Well insulated homes do NOT overheat in summer

Passivhaus approach maintains cool summer temperatures

I've been prompted to prepare this blog posting after Mark Elton highlighted a blog entry by Mel Starrs on a new group of housebuilders calling itself the 'Futures Group'. This group includes Taylor Wimpey, Barratt Homes, Bovis, Redrow, Miller and McCarthy & Stone and they have produced a report that tries to put a brake on the forthcoming improvements required in the UK Building Regulations.

One of the old myths that is being pedalled is the notion that well insulated homes overheat in summer. The report says:

"…concern is being expressed about summer overheating in well insulated new homes. Evidence is anecdotal in the main, with little published, verifiable information available, but studies to date suggest that SAP is inadequate in this area and an approach employing more complex dynamic modeling is required to more accurately assess and understand the mechanisms involved."

Well we have evidence from Technology Strategy Board funded research on several of our buildings, both new build and retrofit, that establishes very clearly that well insulated homes do NOT overheat in summer if it they are designed with either (a) conventional sized south facing windows or (b) large south facing windows with external blinds.

In fact, the graphs taken from our soon to be published report of our Hounslow Retrofit-for-the-Future refurbishment with 240mm of external insulation and other passivhaus features such as triple glazing show that indoor temperatures are in fact COOLER in summer than in winter, and the insulation helps keep the buildings cool.

Far from requiring dynamic modelling, all that is required is the Passivhaus Planning Package as a design tool, and these results are achieved in a building that doesn't even go as far as achieving Passivhaus Certification.

Comments 14

I agree. Our retrofit experience using PHPP & basic design common sense (!) backs this up. I have proof Grove cottage EnerPHit refurb has never overheated (since 2009). Care must be taken of course but as stated already - insulation keeps heat out too, and PHPP warns against overheating + common sense = safe outcomes.

Well insulated homes DO overheat in summer !
No, sorry, that is wrong.
About as wrong as Justin Bere's statement.
Assuming the dwelling achieves "Passive House" criteria (maybe even is PH certified) many different parameters determin whether & how often the internal temperature exceeds 25 degrees Celsius: orientation, shading, ventilation, material (thermal mass),and more. Much of this can be viewed here:
http://passipedia.passiv.de/passipedia_en/basics/summer (incl. heavyweight v. lightweight materials).
Justin Bere is right to say that the PHPP is a very good design tool, which does have a dedicated section to assess the summer performance.
So the question left is: When Wolfgang Feist agrees with Peter Warm that the PHPP 10% overheating rule should be more like 5%, why is it not amended? It would make sense. The interview can be seen here:
http://www.youtube.com/watch?v=tz7pYopcZN4
Christian Nialki

Nialki, good to speak to you today after my presentation for the Technology Strategy Board at the BRE. I think we agree this is all a matter of degree. I love the benefits of the clay materials that you distribute in the UK, and accept that they do have all sorts of benefits to indoor comfort and air quality. However the proven fact (eg Camden Passivhaus, timber frame with plasterboard linings, London 2010), as also predicted by scientific research, is that in a well designed passivhaus we need very little thermal mass to maintain very comfortable indoor conditions in summer and winter. That's great news where we are trying to keep control of costs. However I accept it's a nice luxury to have a little more with a good low embodied material like clay panels where budgets permit. We'll speak soon about using your products on a live project of ours. Regards, Justin

Nialki I haven't checked all your references yet, but: http://passipedia.passiv.de/passipedia_en/basics/summer
you will I think find the answer to your question about the 10% figure. In a professionally designed, well proportioned passivhaus with large areas of south facing glass, even if intuitive and easily understood use of window ventilation and temporary shading are not used by the occupant, summer overheating remains less than or equal to 10%, even in the most vulnerable upper 1st floor room. Our experience from monitoring the Camden Passivhaus (timber frame with plasterboard linings) suggests, reassuringly, that well-designed passivhaus buildings in the UK are equally robust in use regardless of whether they are built from light or heavy materials. Furthermore, optimal results are reliably achieved by intuitive age-old methods equivalent of enjoying a cool night time breeze and enjoying shade from the midday sun.

Dear Justin, Yes, good to speak to you too, and yes, I too think that we agree on many points, of which the most important one is to make Passive Houses a successful as possible.
To make it clear, summer overheating is NOT going to be a problem for the Hounslow retrofit PH.
Solid wall at the inside and 240mm EPS on the outside delivers about an 4.40 W/m2 K thermal admittance and a k-value of 170kJ/m2K. But assuming only 40mm EPS would be fitted to the insite, the figures would be 1.06 thermal admittance and a k-value (areal heat capacity) of only 10.40kJ/m2K, or ca. 2.50Whr/m2K.
Sorry about the numbers, but where would the solar and internal heat gain be stored?
Great that you like our clay building boards. The unfired clay material is about thermal mass, moisture buffering, acoustic insulation, etc., but they are very much about embodied energy / carbon as well. For all constructions, but especially Passive Houses, the CO2 footprint of all building materials needs to be much more accounted for.
A PH build with concrete + EPS (like the AI PassivHaus) is very different to a PH using timber frame, cellulose insulation and internal clay board lining. The embodied energy in the AI PH is probably sufficient to heat them both for 50-60 years, or more. I think that should be a seperate serious discussion, and I know you think so too.
Christian Nialki

Dear Christian,
Whilst I agree with many of the points that you have raised here there are a couple of issues I would like to add to the debate:
Firstly numbers in isolation are meaningless, as Alan and Sophie have pointed out we first need to understand the boundary conditions and dominant variables applying in each context. I have written a short piece about thermal mass optimisation in Passivhaus for Ecotech 24 (Nov 2010) “Thermal mass: is more always better?” which may help put this all in to perspective www.bere.co.uk/sites/default/files/Full%20magazine%20small.pdf. As a general rule of thumb if we can dampen the internal 24h temp swing resulting from the solar gains to approximately 2K or less than we probably have enough thermal mass. The best way to do this is to reduce the solar gains entering the building in the first place, hence glazing ratio optimisation and external shading devices. After first fine tuning the glazing, shading and ventilation strategies, then secondly adding sufficient thermal mass to dampen the internal temperature swing to approx (delta)2K is the icing on the cake.
Assuming we are talking about a passivhaus dwelling or building with low internal gains then there are many ways we can add the relatively small amount of thermal mass that is needed. Two layers of plasterboard, Fermacell, 18mm of OSB, massive timber plate, parge coats (clay, plaster etc) are all possible options. We also have the option to use transient thermal mass (hydraulic systems) like wall or ceiling panels or which can act as both heating and cooling devices or even consider displaced thermal mass, i.e. outside the building envelope but coupled to the supply air system (e.g. GAHE, EAHE or brine/air systems etc).
Before we select the choice of thermal mass we should, as responsible designers, also consider the wider sustainability impacts (embodied energy, embodied carbon, biodiversity, use of non-renewable materials, end of life disposal etc)
Kind regards
Rob

It's no real surprise that it doesn't overheat. External insulation was used which meant that the house had 230 mm of solid brick thermal mass up against the room air. If they'd had to use internal insulation then the results would have been different.

Ian - good to speak to you at the BRE today too. Yes internal insulation would have given a different result, but in maintaining cool summer temperatures, a large amount of thermal mass on this project, whilst very useful, is not the decisive factor!

A well designed building - which a Passivhaus is - is not only well insulated but will take account of orientation, contextual shading, sizes of windows and appropriate shading devices; material finishes (external & internal) and ventilation exactly to perform well in winter and in summer. So the above blog by bere: architects and Nialki's comments are not mutually exclusive at all;(maybe title should read a PH certified home does not overheat to avoid confusion) in fact they are all considered while using PHPP to design a building. While I am not 'wedded' to Passivhaus buildings; a great advantage of a PH certified building is that it cannot have overheating >10% so it is inherently designed with strategies to prevent overheating; while other buildings only have to meet Part L overheating criteria - so could indeed still overheat I guess, as the bar is set lower. For my book I ran lots of PHPP 'simulations' in 2080 medium & high predicted temperatures across different UK locations and good night ventilation combined with good solar shading during the day prevented overheating to be

Sofie I agree with the good list of points 1-5 in your blog entry. Careful modelling of our designs for the passivhaus homes to be built at BRE Watford shows how important insulation will be in summer as well as winter, and by building in pockets for future installation of roller blinds on the south elevation, these south facing houses will be given a very reasonable measure of additional protection against the climatic temperature rises you refer to.

Good points all round - a more accurate title would be "adding high levels of insulation does not cause houses to overheat" - both Justin's and Andy's examples are of existing conventional houses insulated to Passivhaus U-values.
Of course it is also possible to build a well insulated house that does overheat - simply allow to much solar gain in or prevent means to lose excess heat overnight. A passive solar design relying on movable shading risks overheating if the shading is not used; a school designed to use night-vent risks overheating if security concerns mean vents stay shut.
However the issue implied in the "Futures Group" report was that increasing insulation levels would (alone) lead to overheating, as summer heat loss would be reduced, and bere:architects monitoring presented here demonstrates beautifully that this is not the case.

Good discussion.
Only point I would add is that even a Certified Passivhaus that does not rely on moveable shading can overheat.
PHPP does a good job of estimating over heating and even 10% isn't as bad as it sounds as is based on 25C. However the building is treated as a whole so if you have a small bedroom with a big South or West facing balcony door and windows then that room could be unusable at times.
PHPP is a very powerful tool but can't do the design, something I see people forgetting.
Also common to see designers adding moveable shading in PHPP to solve overheating in over glazed buildings but this is very risky strategy in the UK, for homes at least. Different in hot countries where people are used to leaving shutters closed all day to avoid overheating.

Thanks Nick, good points of detail and basically you’re warning against thinking garbage in, gospel out. Thankfully, evaluating humans are still needed! But on the other topic, user behaviour, we are finding from detailed monitoring of our buildings that well-designed passivhaus buildings are surprisingly robust. Of course user behaviour will affect performance, but in a well-designed certified passivhaus it seems to be a small variation either side of perfection!

Dear Nick, You are right to raise the point of how subjectively it is to define what is "too hot". For me 25 Celsius is ok, just, but 10% definitely to often. I am not interested in the percentage over the year, nor in the daily mean indoor temp.. If for many months the late afternoon / evening temperature is closer to 30 Celsius than 25, even with mechanical ventilation and tilted windows, for me that is too warm, too often. The a few times mentioned page "PH in summer" (http://passipedia.passiv.de/passipedia_en/basics/summer) shows in Fig.8 + Fig.19 well how just the choice of building material influences the summer performance.